1. Field of the Invention
This invention relates to liquid crystal display (LCD) devices, and more particularly, to an apparatus and method for generating a set of bias voltages for an LCD driver to drive an LCD panel, which can provide a large current for a voltage divider to provide adequate bias voltages to the LCD driver at the instant when the LCD waveforms are being switched from one state to another, and a small current in other times for reduced power consumption to save energy.
2. Description of Related Art
Liquid crystal display (LCD) devices are digital display devices widely used on digital watches, palmtop game machines, and various other electronic instruments for display of data or graphics thereon.
FIG. 1 is a schematic diagram of a conventional LCD device, which includes an LCD driver 10 for driving an LCD panel 14 to display data thereon. The LCD driver 10 is coupled to a voltage divider 12 consisting of a number of serially connected 100 k.OMEGA. resistors which can divide an external system voltage V.sub.CC into a number of apportioned voltages [V.sub.1, V.sub.2, V.sub.3, V.sub.4 ] which serve as bias voltages for the LCD driver 10 to output a plurality of analog LCD waveforms, including, for example, eight common signals COM1-COM8 and a number of segment signals SEG1-SEG40, to the LCD panel 14. These LCD waveforms represent the data or graphics that are to be displayed on the LCD panel 14.
Typically, the system voltage V.sub.CC is supplied by a battery unit. However, one drawback to batteries is that the output voltage thereof will be constantly decreasing during use. A brand new battery unit having an output voltage of 1.7 V (volt) at the beginning, for example, will be decreased in the output voltage to 1.2 V after a period of use. Therefore, for a battery unit consisting of three serially connected batteries, the output voltage thereof will be gradually decreased from 5.1 V (1.7V.times.3=5.1V) to 3.6 V (1.2V.times.3=3.6V) during the period of use. In a game machine (for example the BRICK GAME machine) having a resolution of 320 dots, the contrast ratio of the LCD panel thereof will be optimal when the output voltage of the battery unit is within the range from 3.8 V to 4.6 V. The contrast ratio will be overly high when the output voltage of the battery units is over 4.6 V during the beginning of use, and then become inadequate when the output voltage of the battery units is below 3.8 V near the end of the life of use.
The system voltage V.sub.CC is divided by the voltage divider 12 shown in FIG. 1 into a number of bias voltages [V.sub.1, V.sub.2, V.sub.3, V.sub.4 ] for the LCD driver 10 to drive the LCD panel 14. By conventional method, a variable resistor V.sub.R is connected in series to the voltage divider 12 for adjusting the magnitude of a DC current I.sub.d (hereinafter referred to as bias current) flowing through the resistors in the voltage divider 12. The resistance of V.sub.R is typically adjusted to a large value, but this will cause the bias voltage I.sub.d to be low. A conventional solution to this problem is to provide an array of capacitors [C.sub.1, C.sub.2, C.sub.3, C.sub.4 ] connected to the nodes where the bias voltages [V.sub.1, V.sub.2, V.sub.3, V.sub.4 ] are produced. These capacitors [C.sub.1, C.sub.2, C.sub.3, C.sub.4 ] can stabilize the bias voltages [V.sub.1, V.sub.2, V.sub.3, V.sub.4 ] and also allow for a large magnitude to the bias current I.sub.d at the instant when the LCD waveforms are being switched from one state to another.
The provision of the capacitors [C.sub.1, C.sub.2, C.sub.3, C.sub.4 ] shown in FIG. 1, however, needs an increased number of I/O pads on the IC chip of the LCD driver to connect, and represents an increase in component cost. For low-priced palmtop game machines, this means an increase in the manufacturing cost, which will cause the products less competitive on the market. Moreover, under the pad limit requirements, the increased number of I/O pads will force the manufacturer to use low-end fabrication processes (such as the 0.8 .mu.m technology) instead of advanced ones (such as the 0.6 .mu.m technology) to fabricate the IC chip of the LCD. In other words, when there is a limit to the number of I/O pads, the feature size of the IC chip cannot be further reduced even though the 0.6 .mu.m fabrication process is used instead of the 0.8 .mu.m technology.
It is, therefore, a primary research effort in the semiconductor industry to find a solution which allows the elimination of the above-mentioned capacitors (i.e., the capacitors [C.sub.1, C.sub.2, C.sub.3, C.sub.4 ] shown in FIG. 1) coupled to the LCD driver so as to reduce the component cost and also allow the use of the more advanced 0.6 .mu.m technology to fabricate the LCD IC chip.
One solution is to lower the resistance of the resistors in the voltage divider so as to raise the magnitude of the bias current I.sub.d flowing through the voltage divider, thus allowing for an adequate level for the bias voltages supplied to the LCD driver. An inadequate level for the bias voltage would cause spikes to occur in the LCD waveforms.
One drawback to the foregoing solution, however, is that the bias current I.sub.d will be excessive that causes unnecessary power consumption and thus a waste of energy. For example, assume V.sub.CC =5V and 100 k.OMEGA. resistors are used to constitute the voltage divider 12, the bias current I.sub.d flowing through the voltage divider 12 shown in FIG. 1 will be EQU I.sub.d =V.sub.CC /(100.times.5) k.OMEGA.=5/500=10 .mu.A (microampere).
However, when 15 k.OMEGA. resistors are used in place of the 100 k.OMEGA. resistors in the voltage divider 12, the bias current I.sub.d will become EQU I.sub.d =V.sub.CC /(15.times.5) k.OMEGA.=5/75=67 .mu.A,
which is significantly much larger than the previous 10 .mu.A current. This large amount of current is not useful for the operation of the LCD driver 10 but wasted instead. This causes unnecessary power consumption.
The provision of capacitors coupled to the LCD driver is also an impractical scheme since it is impossible to provide an adequate capacitance to capacitors in an IC chip which is very small in size. To do this, the size of the IC chip will become large, which is usually not desired.
One practical solution to the foregoing problem is to connect a variable resistor V.sub.R in series to the voltage divider 12 as illustrated in FIG. 1, so as to adjust for a suitable level for the bias voltages. For example, when the output voltage of the battery unit exceeds 4.6 V, the variable resistor V.sub.R can be adjusted until the level of V.sub.LCD is lowered to 4.2 V (which is the optimal level for the LCD driver 10). On the other hand, when the output voltage of the battery unit is below 4.2 V, the variable resistor V.sub.R can be adjusted to zero resistance so as to provide the maximum possible level for V.sub.LCD.
The foregoing solution of using the variable resistor V.sub.R, however, is still not considered a satisfactory one to provide the best adjustment for the bias current. In view of this, an automatic brightness control apparatus for an LCD device is disclosed in ROC Publication No. 231,148. This patent has two preferred embodiments, respectively illustrated schematically in FIG. 2 and FIG. 3. As shown, the patent of ROC Publication No. 231,148 includes a microprocessor 20, a voltage divider 21, a resistor circuit 22, and an LCD panel 23. The microprocessor 20 is a 4-bit unit having a pair of input ports P8.0, P8.1 connected to the voltage divider 21 and a pair of output ports ALCD1, ALCD2 connected to the resistor circuit 22. Further, the microprocessor 20 has a brightness control port VLCD connected via an internal resistor to a voltage source V.sub.DD. The output of the VLCD port is controllable by adjusting the resistance of the resistors R1, R2 in the resistor circuit 22 so as to allow the LCD panel 23 to display data with a desired brightness and contrast.
The foregoing patent, however, has several drawbacks. First, it needs too many I/O ports, including at least the VLCD, ALCD1, and ALCD2, for control of the LCD panel 23. Second, since ordinary IC technology is not able to fabricate the internal resistor with a precise resistance, which might have a deviation as large as twice the desired resistance, the externally connected resistors R1, R2 should be adjusted so as to match the internal resistor. This usually causes inconvenience to the downstream manufacturers who assemble the external resistors to the IC chip. Third, since the power detection means in the patented device is still turned on when it is not in use, energy is unduly wasted. Fourth, since a large bias voltage will cause the LCD waveforms to be reduced in display quality, the patent is not suitable for use on LCDs with large display panels since the pixels therein are each associated with a large capacitance. It is also not suitable for use on LCDs with a large resolution since the number of pixels is large. Fifth, the externally connected resistors not only cause an increase in component cost, but also cause an increase in chip size to the IC chips having pad limit requirements since they take up at least an additional three I/O ports. There exists, therefore, a need for a new apparatus and method for generating bias voltages which can solve the foregoing problems.